Dendritic cells (DCs) are important for both immunity and self-tolerance. Activation of peripheral tolerance mechanisms by administration of tolerizing DCs is proposed as a strategy for the treatment of type 1 diabetes (T1D) and prevention of islet allograft rejection. We have previously demonstrated a defect in the tolerogenic capacity of DCs in NOD mice characterized by persistent IL-12 and minimal IL-10 expression, which may explain pancreatic accumulation of autoreactive T cells in T1D. Forced expression of IL-10 has the potential to restore tolerogenicity to NOD DCs. Moreover, transfer of tolerogenic DCs prevents T1D in NOD mice. Transduction with adenovirus (Adv) containing human IL-10 achieves a DC phenotype with low IL-12 and high IL-10 expression in B6 mice. In preliminary studies, this DC phenotype inhibits Th1 responses in syngeneic and allogeneic MLRs. We have also demonstrated for the first time successful DC transduction with recombinant adeno-associated virus serotype 1 (rAAV1) vectors. In contrast to adenovirus, transduction with this vector does not induce DC activation, as determined by IL-12 production or upregulation of CD86/MHC class II, suggesting virus-specific properties may be important for restoration of tolerogenic function to NOD DCs. The major focus of this proposal is the development of IL-10 producing tolerogenic DCs for the prevention of T1D and islet cell rejection in NOD mice. In these experiments, we will (1) Develop NOD DCs that express hlL-10 via gene therapy with Adv and rAAV1 vectors; (2) Determine the mechanisms by which hlL-10 transduced NOD DCs inhibit autoreactive Th1 responses; and (3) Determine if hlL-10 transduced NOD DC injection prevents the development of T1D and rejection of islet cell grafts in NOD mice. The overall unifying hypothesis is that forced NOD DC expression of hlL-10 will be effective in overcoming the defect in tolerogenic function. In testing our hypothesis, we will determine effects of Adv and rAAV1 transduction on NOD function, independent of the transgene and unique to each virus, which will provide important considerations when designing studies using DCs modified by these vectors. In determining the mechanisms of tolerance induction by IL-10 transduced NOD DCs, we will further the understanding of the complex pathogenesis of T1D. Finally, our studies will be important to the development of clinical strategies designed to prolong the survival of islet cell transplants.